In recent years, most fixed magnetic recorders (HDD: hard disk drive) have improved in various element technologies in response to increasing demand for higher recording density. Decisive elements for determining a recording density of a HDD include a magnetic head, a storage medium, a recording/reproducing channel, and a positioning mechanism. In order to maintain further improvement of recording density in response to the current trend in the future, it is important not only to improve the individual element technologies but also to combine the individual element technologies as a system to examine the entire effect. To be specific, in order to achieve ultra-high density, it is necessary to dramatically improve recording density both in the linear recording direction (BPI) and the radius direction (TPI) of a disk formed like a circular plate.
As to technology of heads, using a GMR (gigantic magnetic resistor) head has successfully improved capability of reproducing a magnetic recording signal and has remarkably contributed to higher density mainly in the linear recording direction of a disk. As to magnetic mediums for recording data read by the GMR head, technology for lower noise, higher coercive force, and so on has improved, and as to an HDI (head disk interface) and a mechanical servo system, technology for reducing a flying height of a head and technology for positioning a head have improved so as to remarkably contribute to higher recording density in the radius direction of a disk.
Unlike a CD-ROM, a DVD-ROM, a FD, etc. having a constant linear velocity (CLV), a hard disk drive is designed with a constant number of revolution (CAV: constant angular velocity). Thus, although an angular velocity is constant at any radius, a linear velocity is different depending on each place. Namely, a linear velocity tends to be larger on the outer circumference than the inner circumference of a magnetic disk. Hence, if it is assumed that data recorded on a magnetic disk has a constant recording frequency at any radius, the outer circumference of the disk is larger in minimum interval of magnetic inversion between each other adjacent pieces of data in the data track tangent direction as compared with the inner circumference, thereby reducing a recording density. Namely, recording is less efficient on the outer circumference as compared with the inner circumference.
In order to correct the problem as much as possible, so-called zone bit recording (ZBR) method has been adopted in which data is divided into some zones (e.g., 16 zones or 8 zones) in a radius direction, a recording frequency is switched for each of the zones to change the number of sectors in the data, and therefore a difference in minimum interval of magnetic inversion between radiuses is reduced. This method is sometimes referred to as constant density recording. This method changes and optimizes a recording density in a linear recording direction for each zone, so that recording efficiency in the BPI (Bit Per Inch) direction can be improved entirely on the device.
However, the transfer rates of existing devices are limited because of the processing speed of a semiconductor chip such as a data channel for performing modulation and demodulation and a hard disk controller for performing serial/parallel conversion. Further, another method is available which reduces the number of revolution of a magnetic disk and sacrifices performance to lower a transfer rate. However, this method is hard to be a positive solution and does not remarkably match the needs of the marketplace. The industry rather intends to increase the number of revolution to raise a transfer rate. Therefore, in the future, it can be expected that a recording density in a radius direction rather than a linear recording direction will be raised to increase a capacity, and therefore the entire capacity will be efficiently increased, which will become an essential condition.